To continuously cast a billet, a molten steel 51 is charged into a casting mold 50, having a substantially square inner section and oscillating up and below from a tundish above the casting mold as shown in FIG. 18, and a solidified shell 52 is formed on the inner surface of the casting mold while heat is absorbed from the side surface of the casting mold 50 which is cooled with water. The solidified shell 52 is then drawn out gradually, and the molten steel 51 at a core portion, too, is gradually solidified so as to form a billet.
To accomplish lubrication between the inner surface of the casting mold and the solidified shell 52, rape seed oil (an example of the lubricant) is charged little by little from above the casting mold 50, and is then carbonized to obtain a lubricant.
However, when casting of the billet is carried out at a high speed (at 3 m/min, for example), a difference in solidification shrinkage occurs because the gap between the solidified shell 52 around the four outer periphery surfaces of the billet and the casting mold 50 is not uniform, and the section of the product becomes a rhomboid. In a round billet, side periphery deformation such as an oval section of the product or the occurrence of a recess takes place. For this reason, the continuous casting method of the billet according to the prior art has been carried out within an allowable speed range in which this rhomboidity, i.e. rhomboid deformation, does not occur, and the problems of a relatively low speed of the casting speed and low productivity are yet left unsolved.
In the case of continuous casting of slabs having a rectangular section, on the other hand, Japanese Examined Patent Publication (Kokoku) No. 57-11735 proposes a casting mold for continuous casting which is directed to prevent longitudinal cracks of a slab and damage such as slag bite by disposing uniformly a large number of recesses having a width or diameter of not greater than 2.5 mm at a part of the inner surface or the whole inner surface of the casting mold. It has been found out that when this technology is applied to continuous casting of billets, the recessed portions are gradually filled with carbon powder as the lubricant because the diameter of the recessed portions is not greater than 2.5 mm, and stable casting cannot be conducted.